Seasonal Changes\nin the Oxidative Potential of Urban\nAir Pollutants: The Influence of Emission Sources and Proton- and\nLigand-Mediated Dissolution of Transition Metals

The inhalation of fine particulate matter (PM<sub>2.5</sub>) is\na major contributor to adverse health effects from air pollution worldwide.\nAn important toxicity pathway is thought to follow oxidative stress\nfrom the formation of exogenous reactive oxygen species (ROS) in the\nbody, a proxy of which is oxidative potential (OP). As redox-active\ntransition metals and organic species are important drivers of OP\nin urban environments, we investigate how seasonal changes in emission\nsources, aerosol chemical composition, acidity, and metal dissolution\ninfluence OP dynamics. Using a kinetic model of the lung redox chemistry,\nwe predicted ROS (O<sub>2</sub><sup>•–</sup>, H<sub>2</sub>O<sub>2</sub>, <sup>•</sup>OH) formation with input\nparameters comprising the ambient concentrations of PM<sub>2.5</sub>, water-soluble Fe and Cu, secondary organic matter, nitrogen dioxide,\nand ozone across two years and two urban sites in Canada. Particulate\nspecies were the largest contributors to ROS production. Soluble Fe\nand Cu had their highest and lowest values in summer and winter, and\nchanges in Fe solubility were closely linked to seasonal variations\nin chemical aging, the acidity of aerosol, and organic ligand levels.\nThe results indicate three conditions that influence OP across various\nseasons: (a) low aerosol pH and high organic ligand levels leading\nto the highest OP in summer, (b) opposite trends leading to the lowest\nOP in winter, and (c) intermediate conditions corresponding to moderate\nOP in spring and fall. This study highlights how atmospheric chemical\naging modifies the oxidative burden of urban air pollutants, resulting\nin a seasonal cycle with a potential effect on population health.